Wireless communication systems use limited frequency resources, and thus a research on a scheme for efficiently using given frequency resources has been actively conducted. Unlike a Frequency Division Multiple Access (FDMA) scheme using a guard band between subcarriers, Orthogonal Frequency Division Multiplexing (OFDM) overlaps bands while keeping subcarriers orthogonal to each other, thereby improving frequency efficiency, and therefore, OFDM has been recently employed as a standard for most communication and broadcast transmission schemes.
One of other methods for improving a data transfer rate is to simultaneously transmit several signals in one frequency resource and control interference caused by such transmission. With a scheme, a transmitter or a receiver can cancel interference by exploiting channel information. An interference control scheme actively studied in recent times is an Interference Alignment (IA) scheme that avoids interference through proper signal processing, instead of canceling interference. The IA scheme theoretically achieves a Degree of Freedom (DOF) of K/2 in an interference channel where there are K users, but a practical IA scheme capable of achieving a DoF of K/2 is not yet to be persuaded.
The following example regards an IA scheme applicable to a restricted case of K=3. The IA scheme can be roughly classified into time-domain, frequency-domain, and space-domain schemes.
According to the time-domain IA scheme, assuming that in an interference channel including three transmitters and three receivers, a desired signal has a time delay of 1 and an interference signal has a time delay of 2, in the first time slot of two independent time slots, each of the three transmitters transmits one different signal to achieve a DoF of 3/2. However, such an assumption is applicable only to a very limited area and thus is not realistic. Moreover, in a real channel, there is multipath delay spread, such that the assumption is difficult to apply to a real communication environment.
According to the frequency-domain IA scheme, in an interference channel including three transmitters and three receivers, (3n+1) data are IA-precoded and transmitted in (2n+1) independent frequency domains, and the three transmitters transmit n, n, and (n+1) data, respectively, such that as n increases, a DoF of 3/2 may be achieved. However, actually, a finite natural number n has to be used, and thus a DoF is smaller than 3/2.
Last, according to the space-domain IA scheme, three transmitters and three receivers have the same M (≧2) antennas, thus achieving a DoF of 3M/2. However, this scheme also has a restriction of requiring the same number of antennas, M for each transmitter and receiver.